M2E: A Multiple-Input, Multiple-Output Function Extension for RISC-Based Extensible Processors

Recent study shows that a further speedup can be achieved by RISC-based extensible processors if the incorporated custom functional units (CFUs) can execute functions with more than two inputs and one output. However, mechanisms to execute multiple-input, multiple-output (MIMO) custom functions in a RISC processor have not been addressed. This paper proposes an extension for single-issue RISC processors based on a CFU that can execute custom functions with up to six inputs and three outputs. To minimize the change to the core processor, we maintain the operand bandwidth of two inputs, one output per cycle and transfer the extra operands and results using repeated custom instructions. While keeping such an limit sacrifices some speedup, our experiments show that the MIMO extension can still achieve an average 51% increase in speedup compared to a dual-input, single-output (DISO) extension and an average 27% increase in speedup compared to a multiple-input, single-output (MISO) extension.